The requirements that a heat-storage material should satisfy include to have a large amount of stored heat, to function at a given temperature level, to be stable over a prolonged period, to be inexpensive, to be non-toxic, to be non-corrosive, etc. As substances that meet these requirements, salt hydrates undergoing a phase change are the most frequently investigated, of which sodium sulfate decahydrate is representative.
Since sodium sulfate decahydrate has a melting point of 32.degree. C. and a latent heat of 60 cal/g, a large number of attempts to utilize it as a heat-storage material have so far been made since 1952 when sodium tetraborate decahydrate (Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O) was found to be effective as a supercooling inhibitor to be used in combination therewith.
A problem encountered in investigations for practical use is that sodium sulfate decahydrate shows incongruent melting. That is, upon melting, anhydrous sodium sulfate forms and precipitates at a bottom of the liquid. When this is cooled, the surface layer of the precipitated anhydrous salt regains water to return to the salt decahydrate but the inner part remains as the anhydrous salt. Since the remaining anhydrous salt does not contributes to the phase change, amount of the stored heat decreses. In order to overcome this, various methods have been investigated for dispersing and holding the anhydrous salt in the liquid without permitting it to precipitate to the bottom of the liquid. In these methods, precipitation is prevented by increasing the viscosity by means of an additive which is either an inorganic compound or an organic polymer.
For example, methods comprising using various inorganic compounds were attempted as described in Unexamined Published International Application Sho-55-501180 and JP-A-53-34687. However, they failed to always achieve the sufficient effect of preventing precipitation. (The term "JP-A" used herein means an unexamined published Japanese patent application.)
Further, although methods comprising using a water-soluble polymer, e.g., poly(sodium acrylate), or a crosslinkable polymer as an organic polymer have been proposed in JP-B-57-30873, JP-B-57-48027, JP-A-58-132075, JP-A-59-102977, etc., these methods have not been always sufficient in long-term stability. (The term "JP-B" used herein means an examined Japanese patent publication.)
In JP-A-60-203687 is proposed a method for inhibiting the decrease of amount of stored heat in a Glauber's salt-based heat-storage material composition by adding a silicone anti-foaming agent and water containing a chelating agent to Glauber's salt. In this method, it is essential that a silicone anti-foaming agent and a chelating agent should coexist, and if both are absent, a decrease of amount of stored heat is observed after 500 heating cycles.
There also is a proposal of a heat-storage material obtained by adding a nucleating agent and a surfactant to a heat-storage material of the latent heat type and temporarily melting the resulting heat-storage material to uniformly disperse the nucleating agent (JP-A-62-297381). There is a description therein to the effect that since the nucleating agent is prevented from precipitating and is evenly dispersed due to the addition of a surfactant, solidification takes place homogeneously and the melted heat-storage material has intact flowability, so that workability is not impaired. However, because the nucleating agent dispersed in this manner is kept in the dispersed state by the electrostatic repulsion attributable to the surfactant, gravitational sedimentation during use over a prolonged period of several months or several years is unavoidable and it is difficult to maintain the uniformly dispersed state of the nucleating agent.
The present inventors, under these circumstances, previously proposed a heat-storage material composition and a process for producing the same, which composition does not undergo a decrease of amount of stored heat over a prolonged period due to the use of a crosslinked polymer obtained by polymerizing at least one monomer selected from an unsaturated carboxylic acid, an organic unsaturated sulfonic acid, and salts of these with a polyfunctional monomer (JP-A-62-25188).
On the other hand, in heat-storage materials for cooling in air conditioning, heat storage in ice has attracted attention for long and has come into practical use. This utilizes the great latent heat of fusion of ice (80 cal/g). However, there is a problem that cooling to -10.degree. C. or below is necessary for ice production and this necessitates a special freezer, resulting in a heavy initial investment in equipment.
With respect to heat-storage materials comprising sodium sulfate as the main ingredient, investigations have been made on melting point regulators in order to lower the melting point to around 10.degree. C. In U.S. Pat. No. 4,689,164 is proposed a system composed of a composition comprising sodium sulfate decahydrate and, per one mole of that salt, 3/4 to 6/4 mole of ammonium chloride and 1/5 to 1/4 mole of potassium chloride and, further including, a thixotropic agent and a nucleating agent. There is a description to the effect that this system has a melting point of 50.degree. F. (10.degree. C.) or below and a heat of fusion of 38-42 BTU/lb (21-23 cal/g). This system, however, has a problem that the melting point is so apart from the solidifying point that it should be cooled to 3.degree. C. or below for solidification.
Accordingly, an object of the present invention is to overcome the problems of the prior art techniques described above. That is, the present invention provides a heat-storage material composition which undergoes a phase change between solid and liquid phases in the range of working temperatures for freezers generally employed for cooling in air conditioning (6.degree.-14.degree. C.), and which is free from a decrease of amount of stored heat over a prolonged period.